This invention relates generally to thermostat control of heating, and more specifically to means for lowering the effective trip temperature of a high limit thermostat and for raising its effective reset temperature, all relative to the temperature of a heat source to be controlled.
A high limit thermostat is required for electric spa heaters (by Underwriters Laboratories Inc.), to insure that if the adjustable water temperature regulating thermostat fails, the maximum temperature that the water can attain will be within a relatively safe range. The first standard written by UL established that temperature to be 158.degree. F. The standard required that the high limit thermostat must trip at a temperature lower than that maximum; it must break both sides of the line; it must have a manual reset arrangement; and it must be independent of the adjustable regulating thermostat. In addition, the manual reset mechanism must be "trip free", meaning that if the reset button is held manually, it will not prevent the thermostat from breaking the circuit if it reaches its trip temperature. A thermostat to meet the above high limit requirements is known in the art, one example being Therm-O-Disc, Inc. model HL.
A later standard imposed by UL for electric spa heaters used with spa equipment reduced the maximum allowable water temperature to 122.degree. F. This lower temperature considerably increased the problems involved in providing a suitable thermostat, especially for high wattage heaters in the 11.5 KW range. The larger the current needed to be switched, the larger would be the differential between the thermostat trip temperature and its reset temperature. These thermostats use a bi-metal disc as a sensing and contact activation means, the disc snapping over center like the bottom of an oil can. At the 122.degree. F. maximum requirement, the temperature that the disc must be lowered to, to cause it to reset, is in the 60 to 70 degree F. range. If the thermostat should trip in warm weather, it requires special cooling techniques to cause it to reset. Since the trip tolerance available is, at best, .+-.5.degree. F., the trip temperature can be as low as 112.degree. F., and the reset temperature in the 60.degree. to 80.degree. F. degree range, At that low trip temperature, a heater sitting unused in the heat of the sun can become hot enough to cause the thermostat to trip.
One expedient that can be resorted to in answer to this problem is to use a low current thermostat which has a low differential to sense the temperature. Such a thermostat could be used to de-activate a contactor which is capable of switching the higher current levels and can be connected with a latching circuit to make it manually resettable. The disadvantages to this system are that the required contactor is large and expensive, and the additional reset switch is required. It also must be manually reset every time the power is interrupted. There is a need for a low cost, simple solution to these discussed problems.